1. Field of Invention
This invention relates to methods for forming surface micromachined structures to make fluid ejection devices.
2. Description of the Related Art
Polysilicon surface micromachining uses planar fabrication process steps common to the integrated circuit (IC) fabrication industry to manufacture microelectromechanical or micromechanical devices. The standard building-block process consists of depositing and photolithographically patterning alternating layers on a substrate. The alternating layers consist of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material such as silicon dioxide on a substrate. Vias etched through the sacrificial layers provide anchor points to the substrate and between the polysilicon layers. The polysilicon layers are patterned to form mechanical elements of the micromachined device. The mechanical elements are thus formed layer-by-layer in a series of deposition and patterning process steps. The silicon dioxide layers are then removed by exposure to a selective etchant, such as hydrofluoric acid (HF), which does not attack the polysilicon layers. This releases the mechanical elements formed in the polysilicon layers for movement thereof.
The resulting micromachined device generally consists of a first layer of polysilicon which provides electrical inter-connections and/or a voltage reference plane, and up to three additional layers of polysilicon which include mechanical elements ranging from simple cantilevered beams to complex systems, such as an electrostatic motor connected to a plurality of gears. Typical in-plane lateral dimensions can range from one micron to several hundred microns, while the layer thicknesses are typically about 1-2 microns. Because the entire process is based on standard IC fabrication technology, hundreds to thousands of devices can be batch-fabricated, fully assembled (without any need for piece-part assembly) on a single silicon substrate.
A chemical mechanical polishing (CMP) technique that planarizes the various levels in a multilevel micromachined device to prevent unintended interference between structures on different layers of the micromachined device is described in U.S. Pat. No. 5,804,084 to Nasby et al. In the above-described process, as the relatively thick (2 xcexcm) layers of polysilicon and oxide are deposited and etched, considerable surface topography arises which imposes limitations in deposition, patterning and etching of subsequent layers. The topography is created as the layers are draped into valleys created by prior etching steps. It is often desirable to planarize specific layers in order to eliminate processing difficulties associated with photoresist step coverage, depth-of-focus of photolithography equipment, and stringer generation during dry etch. The chemical mechanical polishing of intermediate sacrificial layers as disclosed in U.S. Pat. No. 5,804,084 provides relatively simple and quick processing to ameliorate the topography difficulties inherent in multi-level micromachining processes. This avoids the need for additional care in design of structures, special photoresist processes and the use of extra mask levels.
An anisotropic etching process may be used to define structures, for example trenches and ridges or the like having low to average selectivity, into silicon substrates. Individual structures to be etched in are usually defined by etching masks applied to the silicon substrate by way of so-called masking layers, for example, a photoresist layer. In the anisotropic etching technique, it is necessary to achieve a laterally exactly defined recess in the silicon. These deeply-extending recesses must have lateral ends which are as exactly vertical as possible. The edges of the masking layers covering those silicon substrate regions that are not supposed to be etched are not underetched in order to keep the lateral precision of the structural transition from the mask into the silicon as high as possible. As a result, it is necessary to allow the etching to progress only on the bottom of the structures, but not on the already produced side walls of the structures.
To this end, a plasma-etching method may be used to perform etching of profiles in silicon substrates. In such a method, chemically reactive species and electrically-charged particles (ions) are generated in a reactive gas mixture in a reactor with the aid of an electric discharge. The positively-charged cations generated in this manner are accelerated toward the substrate, by means of an electrical prestress applied to the silicon substrate, and fall virtually vertically onto the substrate surface, and promote the chemical reaction of the reactive plasma species with the silicon on the etching base.
A particular type of anisotropic etching process is described in U.S. Pat. No. 5,501,893 to Laermer et al. This particular type of etching process is commonly referred to as a Bosch etch. According to a Bosch etch, the anistropic etching process is achieved by alternating sequential etching and polymerization steps. As a consequence, in an advantageous manner the simultaneous presence of etching species and polymer formers in the plasma is avoided altogether. Hence, deep structures having vertical edges can be realized with very high etching rates in silicon substrates.
This invention provides methods for forming surface micromachined fluid ejection devices using a two-step etching process. In various exemplary embodiments of the methods of this invention, the two-step etching process comprises a modified Bosch etch that is used to create a structure through which fluid can be brought into an ejection chamber of the fluid ejection device that is defined on a layer, for example, a silicon wafer.
In various exemplary embodiments of the modified Bosch etch according to the methods of this invention, a first mask that defines at least one large feature is formed over a surface of the layer. The first mask is then treated to render the mask inert to a chemical rinsing agent. A second mask that defines at least one small feature is formed over the first mask. The at least one small feature is then etched into the layer. The second mask is removed using the chemical rinsing agent after the at least one small feature is etched. The at least one large feature is then etched into the layer whereby the at least one small feature propagates ahead of the at least one large feature.
In various exemplary embodiments of the modified Bosch etch according to the methods of this invention, the two-step etch is propagated from a back side of a silicon wafer to a front side of the wafer. When the etch reaches the front side of the wafer, the etch stops on a first layer of sacrificial material, such as an oxide, associated with a surface micromachining process. The sacrificial material layer(s) on the front side of the wafer are removed during a release etch, such as a hydrofluoric (HF) etch, to allow the polysilicon structures of the fluid ejector to move, and to open up the Bosch-etched structure in the wafer to create a path through which fluid may enter the ejection chamber of the fluid ejector.
The present invention separately provides methods for forming a surface micromachined device in which a stiffening feature is formed on a substrate-facing surface of a polysilicon layer of the device. In various exemplary embodiments of such methods, at least one cut is formed in a sacrificial material layer such that the at least one cut does not extend completely through the sacrificial material layer. A polysilicon layer is formed over the sacrificial material layer. The sacrificial material layer is then removed such that at least one feature corresponding to the at least one cut is formed on a substrate-facing surface of the polysilicon layer. In various exemplary embodiments, the at least one feature on the substrate-facing surface of the polysilicon layer provides a stiffening rib-like structure, for example, that maintains rigidity of a piston structure formed in the polysilicon layer as the piston structure moves to eject a drop of the fluid.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the methods and devices according to this invention.